This invention generally relates to the direct chill (DC) casting of light metal products such as aluminum and aluminum alloys in an electromagnetic (EM) field.
In brief, the DC casting process comprises introducing molten metal into the feed end of the open ended passageway of a tubular shaped mold, solidifying or partially solidifying the molten metal as it travels through the pasageway and applying coolant to the surface of the solidified or partially solidified metal as it emerges from the discharge end of the open ended passageway. At the start of the cast a bottom block or other device is disposed at the discharge end of the mold to block off the passageway, and, when the mold passageway is full of molten metal and the metal therein is sufficiently solidified to support itself, the bottom block is gradually withdrawn from the discharge end. Although coolant is applied to the backside of the mold body to cool the molding surfaces thereof and to thereby initiate the solidification of molten metal within the mold bore, most of the solidification is effected by the application of coolant onto the surface of the cast metal as it emerges from the discharge end of the mold. This is due to the fact that, when the molten metal contacts the water cooled, chill surfaces of the mold bore, it forms an initial shell or embryo and the metal stream then contracts and pulls away from the mold surfaces as solidification proceeds. Once contact with the metal stream is lost, very little heat transfer is effected through the mold walls.
In conventional DC casting a curtain of coolant is applied completely around the periphery of the emerging metal and generally at an angle of about 5.degree. to 30.degree. from the metal surface and in the direction of metal movement. The coolant is applied at a shallow angle to minimize the splashing of coolant from the metal surface which can disrupt the transfer of heat and have a detrimental result on the solidification rate.
Electromagnetic (EM DC) casting is a modification of the conventional DC casting process in which electromagnetic forces are employed to control the shape of the molten metal as it solidifies rather than the bore of the conventional, tubular shaped DC casting mold. In most respects EM DC casting is essentially the same as conventional DC casting, except that there are no chill surfaces in the EM DC casting process to initiate solidification. Essentially all of the cooling for solidification is effected by the application of coolant onto the surface of the metal as it emerges from the discharge end of the EM casting assembly. For further information on EM DC casting, see U.S. Pat. Nos. 2,686,864 (Wroughton et al), 3,605,865 (Getselev), 3,646,988 (Getselev), 3,985,179 (Goodrich et al) and 4,004,631 (Goodrich et al).
Both conventional DC and EM DC casting have various coolant flow requirements for efficient and effective casting depending upon, among others, the size and shape of the ingot or billet, the alloy composition and the surface characteristics of the ingot or billet emerging from the mold. Additionally, coolant flow requirements at the start of a cast may be considerably different than those for the remainder of the cast. Even during casting, the coolant requirements may change due to changes in the casting rate or surface characteristics of the ingot or billet.
Various techniques have been used over the years to control the application of coolant on the surface of the ingot or billet emerging from the discharge end of the DC casting mold. In U.S. Pat. Nos. 2,791,812 air jets are directed into the coolant stream to atomize the liquid coolant before it contacts the mwetal surface to thereby prevent liquid coolant from flowing on the ingot or billet surface. In U.S. Pat. No. 3,713,479 coolant flow is reduced at the discharge end of the mold to retard the solidification rate and then a second coolant stream is applied at some distance away from the discharge end of the mold to compete solidification. As shown in FIG. 3 of this reference, the coolant flow to the ingot or billet surface is reduced by directing the coolant stream parallel to the ingot or billet surface then periodically pulsing a fluid such as water or air onto the curtain of coolant to change the direction thereof onto the ingot or billet surface. In U.S. Pat. Nos. 3,623,536 liquid coolant applied to the surface of the metal causes air to be aspirated and mixed with the liquid coolant to retard its cooling properties. In U.S. Pat. No. 3,765,493 the coolant applied to the metal surface at the start of casting is pulsed to retard the cooling effects of the liquid coolant to prevent the cracking characteristic of some aluminum alloys. In German Pat. No. 932,085 a DC casting mold is described in which the coolant flowing on the back side of the mold, parallel to the metal flow, is combined with the coolant from a second coolant stream so that the combined streams can then be applied to the cast metal surface. L. G. Berezin et al in Tsvetnye Metally, 1974, No. 4, pages 56-7, describe the use of three separate coolant application zones in EM DC casting in order to cast thick ingots. While many of these ideas have merit they have not been widely used in commercial DC casting processes.
The start-up of EM DC casting has been characterized by unique problems in that icicle-type appendages are formed on the butt end of the ingot or billet due to the inability of the magnetic forces and the bottom block to completely contain molten metal during the initial start-up period. Small molten metal streams flow over the butt end and solidify, thus forming the icicle-like appendages. These icicle formations require an excessive amount of the butt to be cropped off before further processing can beconducted on the ingot or billet, which adds considerably to the cost of EM DC casting and also severely limits its use.
It is against this background that the present invention was developed.